The invention relates to an adaptive radio system and a method for assigning transmitting and receiving branches in a radio system. The radio system comprises a plurality of mobile stations and at least one base station. In the radio system, at least the base station comprises a plurality of independent transmitting and receiving branches to transfer signals between the base station and the mobile stations.
FIG. 1 in the accompanying drawing shows a simplified block diagram of the UMTS (Universal Mobile Telecommunication System). A mobile station (MS) communicates over the radio path with a base transceiver station (BTS), in the case of FIG. 1, with BTS1. The base station sub-system (BSS) consists of a base station controller (BSC) and base stations (BTS) under its control. A mobile services switching centre (MSC) usually controls a plurality of base station controllers BSC. The MSC communicates with other MSCs, and via a gateway mobile services switching centre (GMSC), the UMTS network is connected to other networks such as the public switched telephone network PSTN, another mobile communications network PLMN or an ISDN network ISDN. The mobile communications system shown by the figure is e.g. the pan-European GSM system or the UMTS system (Universal Mobile Telecommunication System). The GSM system is implemented with the TDMA technology (Time Division Multiple Access) and the UMTS system is proposed to be implemented with the time division multiple access technology (TDMA) or with the code division multiple access technology (CDMA) or a combination of these two, i.e. a so-called hybrid system.
In digital radio systems implemented with the TDMA technique, such as the GSM or the UMTS system, a group of mobile stations MS may, according to the time-division principle, use the same carrier frequency i.e. radio channel for communication with the base station BTS. The carrier is divided into successive frames that are further divided into timeslots, e.g. 8, 16 or 64 timeslots, that are allocated to users as required. From the network point of view, one carrier wave can consequently be used to establish e.g. 8, 16 or 64 traffic channels. In e.g. the GSM system, the channel width is 200 kHz and in the UMTS system 1.6 MHz, according to a proposal.
Code division multiple access CDMA radio systems are based on spread spectrum communication. The data signal to be transmitted is multiplied by a special hash code assigned to the subscriber, whereby the transmission spreads out onto the broadband radio channel, which is e.g. 1.25; 6.4 or 20 MHz. This means that the same broadband radio channel may be used by several users for simultaneous transmission of CDMA signals processed with different hash codes. Therefore, the unique hash code of each subscriber in CDMA systems produces the traffic channel of the system in the same sense as the timeslot does in TDMA systems. At the receiving end, the CDMA signal is despread by the subscriber""s hash code, whereby a narrowband data signal is obtained. At the receiver, the other subscribers"" broadband signals represent noise to the desired signal.
In radio systems, it is of prior art to use so-called SDMA technology (Space Division Multiple Access) employing adaptive antennas, in which subscribers can be distinguished on the basis of their geographical direction as seen from the base station, when e.g. antenna lobes are adjusted at the base station to desired directions according to the location of the mobile stations. Employing SDMA technology thus improves the signal to interference ratio C/I or the signal to noise ratio S/N of the radio connection between the base station BTS and the mobile station MS whereby the overall capacity of the system increases. SDMA technology further improves the frequency efficiency of the network as the transmission is beamed to a narrow geographical area, which means that the same frequency can be reused for another connection in another direction, possibly even at the same base station. Due to the antenna lobe aimed at the subscriber, the disturbing effects of multipath propagation on the radio link are reduced.
The SDMA technology is based on using a number of parallel receivers and transmitters on the same radio connection, e.g. for a base station transmission and reception, and signal processing that generally is performed digitally. The improvement in the signal to interference ratio and/or signal to noise ratio is usually the better the larger number of parallel transmitting and receiving branches are in use. So-called adaptive antennas, i.e. antennas having variable directional pattern, and processing of received signals are used to monitor mobile stations that communicate with the base station via a radio connection, for example by beaming the base station antenna lobe to a narrow geographical area with some prior art method, for example as regards CDMA systems, the antenna lobe can be beamed to the direction or directions from which the best signal components of the connection in question are received.
An adaptive antenna is comprised of an antenna array consisting of several antenna elements, the directional pattern of the antenna array being dynamically changed by phasing the signals of the antenna elements. Such an antenna array may consist of omnidirectional or directional antenna elements that may be arranged e.g. in a linear or planar manner. Signals arriving from outside the main lobe of an adaptive directional antenna are attenuated in proportion determined by the antenna directional pattern in relation to the signals received from main lobe. When adaptive antenna are utilized, the selected mobile station MS may transmit its signal at lower transmit power than normally due to the better antenna gain and signal processing at reception. The transmit and receive antennas may be separate, or the transmission and reception may be arranged to be carried out via a common antenna by separating the receiving and transmitting circuit from one another with a duplex filter.
Adaptive antennas contribute to lower level of interference due to the narrow antenna lobe as interference from other subscribers is reduced and interference caused to other subscribers is lower. Simultaneously, the coverage of the base station is increased towards the antenna main lobe. The antenna signal transmitted by a single mobile station MS can also be received via more than one antenna lobe, in which case signal components caused by e.g. multipath propagation can be combined or the signal from the mobile station, received at different antennas, can be processed in other ways, e.g. by weighting the received signals differently so that the signal is amplified in comparison to other signals that have been received. The directional pattern of an adaptive antenna can thus be changed by receiving a signal via more than one fixedly directed antenna lobes and by suitably weighting the signals thus received.
FIGS. 2a and 2b illustrate a block diagram for receiving and transmitting section of a base station employing SDMA technology. In the example of the figures, four parallel receiving and transmitting branches are tuned to the same channel CH1 and frequency f1. In the SDMA implementation, it is possible to set up as many parallel transmitting and receiving branches as the particular system requires. By increasing the number of transmitting and receiving branches set up in parallel, the antenna lobe can be directed to a narrower geographical area and at the same time to extend the coverage area further from the base station. FIG. 2a illustrates the structure of a base station consisting of four parallel receiving branches. In the block diagram of the receiving section in FIG. 2a, all four receiving branches have a common signal processing unit 201 in which the signals are processed as desired so that just one processed signal CH1 is conveyed from the signal processing unit 201 to other parts of the base station. The structure of one of the receiving branches of FIG. 2a is described more thoroughly in the following. The receiving branch receives a radio signal from the desired direction by means of antenna 202 on frequency f1. The received signal is conveyed to band-pass filter 206 and amplifier 210. Next, the signal is converted to a lower frequency in mixer 214 by multiplying it with the output frequency of local oscillator 218. The converted signal is conveyed via bandpass filter 222 to A/D converter 226 for conversion from analogue to digital form. The signal thus modified is conveyed to the common signal processing unit 201 of the receiving branches, in which it can be processed as desired. The structure of the other receiving branches of the base station is similar to that which has been described in the above.
FIG. 2b correspondingly illustrates the structure of a base station consisting of four transmitting branches. All four transmitting branches of FIG. 2b share a common signal processing unit 235 to which the transmitted signal CH1 is conveyed from other parts of the base station. The signal is processed in signal processing unit 235 as desired and the transmitted signal is conveyed to the four transmitting branches, of which the structure of one is described in more detail in the following. The transmitting branch comprises D/A converter 239 in which the signal is converted to analogue form. The analogue signal is conveyed to mixer 243 in which it is multiplied by the output frequency of local oscillator 247 and thus converted to the transmitted radio frequency, to frequency f1 in the case of FIG. 2b. The converted signal is conveyed to antenna 259 via amplifier 251 and band-pass filter 255. From the antenna, the signal is transmitted to the radio path. The structure of the other transmitting branches of the base station is similar to that which has been described in the above.
FIG. 3 shows the directional pattern of the antennas of the base station units disclosed above in FIGS. 2a and 2b. FIG. 3 shows the directional pattern for reception of the antenna array consisting of four antenna elements 202-205 of FIG. 2a, or the directional pattern for transmission from the antenna array consisting of four antenna elements 259-262 of FIG. 2b, so that the horizontal axis shows the azimuth "THgr" and the vertical axis shows amplitude A. The amplitude and beam width of the directional pattern depends on e.g. the number of antenna elements in the antenna, so that the more antenna elements there are in the antenna, the narrower is the beam and the higher the amplitude which is possible to obtain with the antenna to the main lobe""s direction. Thus, the directional pattern of FIG. 3 shows a narrower beam width and higher amplitude towards the main radiation direction than the directional pattern of a single antenna element.
The problem with prior art systems employing adaptive antennas is that the combination of a plurality of parallel transmitters and receivers required for their maximum performance is expensive to implement and consequently only rarely worth while. In addition, efficient signal processing during normal load on the network is not always necessary, it just contributes to too good a connection to a limited set of mobile stations. The equipment required by SDMA applications are large, which means that they take up a lot of space at the installation site.
It is an objective of the present invention to implement an adaptive radio system in which as small as possible an equipment configuration can simultaneously offer good enough quality of service to as many subscribers as possible.
This object is achieved with a radio system of the type described in the preamble, which according to the invention is characterized by that which is disclosed in the independent claim 1. The particular embodiments of the invention are disclosed in the dependent claims.
The invention further relates to a method for assigning transmitting and receiving branches in a radio system which according to the invention is characterized by that which is claimed in the independent claim 7.
The invention is based on the idea that independent transmitting and receiving branches are dynamically assigned in the radio system to transfer of signals between a base station and a mobile station so that adequate signal quality is obtained for as many subscribers as possible on each traffic channel and control channel. Dynamic assigning of transmitting and receiving branches may thus be utilized for locally increasing the network capacity. Consequently, the equipment configuration designed for maximum performance of the radio system can be employed as efficiently as possible when maximum performance is not required in the radio system. With the inventive radio system, signal transfer does not require too efficient an equipment configuration, but the transmitting and receiving branches not needed in the transfer of the signal in question can be used for transfer of other signals in the system. For transfer of each signal in the inventive adaptive radio system, at least the number of parallel transmitting and receiving branches required by the signal are connected. This dynamic assigning of transmitting and receiving branches creates antenna lobes that change as per connection, or as per timeslot or channel type in e.g. the TDMA system.
The invention is most advantageous when the mobile communications system is based on the time division principle and when the modulation transmit bandwidth may vary subscriber and/or channel specifically, for example in the so-called varying frequency band multirate system, as in the possible GSM/UMTS combination.
Such an adaptive radio system provides the advantage that the capacity of the base station can be increased without adding receivers or transmitters into the equipment. In such a case, the base station is able to serve more mobile subscribers at the same time.
Further, the inventive radio system provides the advantage that, if need be, a channel with better quality than normal can be offered connection specifically for some subscribers.
In addition, the inventive radio system provides the advantage of the system being reliable in equipment failure situations as the equipment can flexibly be reconfigured.